1. Field of the Invention
The present invention relates to method and apparatus for the stabilization of underground excavations using steel tendons or rods, and particularly to tendons which yield rather than break under increased tension.
2. Related Art
Tunnel walls can be stabilized using supporting elements such as timber, structural steel, or rock anchors. A rock anchor is installed into a hole drilled into the rock and typically includes a stiff rod or tendon (usually made of steel), which is affixed to the rock face with a nut and a retaining plate. The rock anchor is fastened inside the rock by mechanical means in contact with the rock, or by using chemical or concrete grouts. See for example, U.S. Pat. Nos. 3,602,000; 3,695,045; 3,967,455; 4,011,787; 4,516,886; 4,564,315; 4,662,795; 4,704,053; 4,954,018; 4,984,937; 5,222,835; 5,233,730; 5,375,946; 5,556,233; 5,791,823; 5,882,148; and 6,030,151, and South African Patent Application No. 90/4879, the disclosures of each being incorporated herein by reference. To install a rock support tendon, the rock hole is first drilled and then the tendon is inserted into the hole and anchored therein using a mechanical shell, a chemical grout, or a cement-based grout.
Known chemical grouts include polyester and latex resins which can be packaged in cartridge form so that they can be inserted into the rock hole and broken and mixed therein using the tendon. When using cartridged chemical products, the product components (adhesive and catalyst) must be mixed together in the process of installing the tendon in the hole to cause the anchoring material to set. The mixing of the product components is usually performed by rotating the tendon such that the roughness or corrugations of the tendon (e.g., the striations on a rebar rod) mix the components. Special mixing devices such as helical coils may be assembled onto the tendon uphole of the anchor in order to provide better mixing quality. See for example U.S. Pat. No. 4,704,053.
However, a problem with such rock anchors is that underground tunnel walls can shift and converge when ground conditions change. Stiff rock anchors, even when subjected to small displacements, will break. It is preferable for the anchor to yield slightly while maintaining its integrity, in order to maintain support of the tunnel walls. Yielding tendons are known which are designed to have some mechanism of yield, so that the tendon cannot break as the rock around the tunnel deforms, and preferably maintains a well-defined and constant load. The yielding tendon support is used in civil mining and tunneling. The yielding tendon is a rock anchor, or a rock bolt that yields when subjected to displacement, but provides resistance to the displacement.
Known yielding tendon support designs are mostly based on frictional pulling resistance mechanisms downhole in the bore or uphole at the tendon head. For example, tendon threads may be designed to yield under stress, allowing a nut or clamp to move with respect to the tendon. Other deformable structures may be provided either downhole or at the tendon head. See for example, U.S. Pat. Nos. 3,967,455; 5,791,823; and 5,882,148. Yielding mechanisms at the tendon head offer a limited yielding displacement range, insufficient for coping with large bursts of energy, induced by mine production blasting or seismic events. Yielding mechanisms based on frictional pulling resistance can perform better in bursting ground, but are expensive and susceptible to corrosion where ground water is acidic.
COMRO introduced the Cone Bolt in 1992, a groutable tendon equipped with a cone anchor. For the Cone Bolt, energy dissipation is achieved when a wedge located downhole at the grouted end of the tendon plows through the filling material confined in the borehole, until the force on the face is no greater than the residual strength of the tendon-grout-rock hole system. The Cone Bolt can sustain slow or rapid convergence of tunnel walls. See Jager, A. J. xe2x80x9cTwo New Support Units for the Control of Rockburst Damagexe2x80x9d, Proc. Rock Support in Mining and Underground Construction, Balkema, Rotterdam (1992), pp. 621-631, and South African Patent Application No. 90/4879. The Cone Bolt was originally designed for use in cement grout. However, it is inconsistent when used with packaged resin due to its inability to mix the resin properly.
Thus, there is a need for a yielding tendon which is capable of sustaining shocks and slow or rapid convergence of tunnel walls. Depending on the selected geometry of the anchor, it can be pre-tensioned and used as active rock support.
It is an object of the present invention to provide a new yielding tendon apparatus and method which overcome the shortcomings of the prior art, and provides a reliable and strong rock anchor capable of withstanding great amounts of shock and load without catastrophic failure, thus enhancing mine safety.
According to a first aspect of the present invention, a yieldable tendon for use in a tunnel includes a rod, a conical wedge disposed at a distal end of the rod with a wider portion of the conical wedge being at a distal end thereof, and a grout mixer protruding from the distal end of the conical wedge.
According to another aspect of the present invention, a yieldable tendon for a tunnel wall hole includes a rotatable rock anchor, and a conical restraining member coupled to a distal end of the rock anchor, the conical restraining member having a cone angle of between substantially 1 degree and substantially 8 degrees with the wider dimension at a distal end of the conical restraining member. An outside diameter of a base of the conical restraining member is smaller than an inside diameter of the tunnel wall hole to permit grout to pass from a downhole portion of the conical restraining member and an uphole portion thereof. The conical restraining member is dimensioned to move through crushed solid grout when a yielding tension is applied to the rod. A grout mixer is disposed on a distal end of the conical restraining member and has a planar surface.
According to yet a further aspect of the present invention, a yieldable rock anchor comprises a metal support member having an outside diameter which is less than a diameter of a rock hole. A wedge anchor is disposed at a distal end of the metal support member and has a narrow portion disposed uphole from a wider base portion thereof. The wedge anchor base portion is narrower than the diameter of the rock hole to permit un-solidified grout to pass from downhole to uphole of the wedge anchor base portion. The wedge anchor is dimensioned to crush solidified uphole grout and permit downhole movement of the crushed solidified grout when a yielding tension is applied to the metal support member and the wedge anchor moves uphole. A grout mixer is disposed at a distal end of the wedge anchor and has a first edge for penetrating a grout cartridge and a second edge for mixing the grout.
According to yet another aspect of the present invention, a rock wall reinforcing kit includes at least one grout cartridge dimensioned to be placed downhole in a rock wall hole. A metal support member is provided and is dimensioned to fit in the rock wall hole. A wedge-shaped anchor is also provided and is coupleable to a distal end of the metal support member so that a wider portion of said wedge-shaped anchor is disposed downhole. The wedge-shaped anchor has a base end dimensioned to permit un-solidified grout from the grout cartridge to pass between sides of the rock wall hole and the anchor base to uphole of the base. The wedge shaped anchor has a wedge angle dimensioned to cause, as a yielding tension is applied to the metal support member, (i) grout uphole of said anchor base to break and move downhole of the base, and (ii) the anchor to move uphole through the grout. A grout mixer is also included and is coupleable to a distal end of the wedge-shaped anchor.
In a further aspect of the present invention, a method of installing a yieldable tendon in a rock hole comprises the steps of: (i) inserting at least one resin cartridge into a downhole portion of the rock hole; (ii) inserting a metal rod into the rock hole, the metal rod having a cone-shaped anchor affixed to a distal end thereof, with the wider base portion of the anchor disposed on the downhole side thereof, a resin mixer disposed on a downhole side of the anchor; (iii) puncturing the resin cartridge with the resin mixer; (iv) rotating the rod to cause the resin mixer to mix the resin; (v) moving the rod further downhole to cause the resin to pass the anchor base portion and move uphole thereof; and (vi) waiting until the resin uphole of the anchor base portion solidifies. Preferably, a nut and a retaining plate are then affixed to the near end of the rod to attach the anchor to the rock face.
Thus, a yielding tendon rock support according to the present invention will more readily be able to provide the following functions:
passive rock carrying effect, produced by the transfer of load through the rock mass in the zone of an originating rock arch;
active stabilizing effects, resulting in stress alteration in the neighbourhood of the mine opening and in the strain state of the rock; and
energy absorbing effect, due its inherent ability to sustain impact loading by transferring part of the impact energy in the destruction of the grout material.